Shellder is a black pearl-like Pokémon encased within a spiky, blue-violet bivalve shell. It has two round white eyes with tiny black pupils. It has an enormously long red tongue, which it uses to burrow itself into sand and to lure and capture prey. Its body is known to be soft and tender; however, its shell is said to be harder than diamond. Prehistoric Shellder were apparently preyed upon by Omastar. Once ensnared by Omastar's tentacles, its hard shell stood no match to the Spiral Pokémon's deadly powerful bite, and its soft insides quickly devoured.

Shellder can develop a symbiotic relationship with Slowpoke. After attaching itself to the Dopey Pokémon's tail, the two undergo a metamorphosis and together evolve into Slowbro. If it attaches to the head of a Slowpoke in possession of a King's Rock, the two will instead evolve into Slowking. These relationships allow it to travel onto land and obtain a new stage in its life cycle.

Regardless of which form the two Pokémon evolve into, Shellder's shell transforms into a spiny, spiraled cone with several tiers. Sharp teeth develop around the rim of the shell to anchor it securely to the evolved Slowpoke, and its eyes now peek out from the bottom rim of the shell. In its crown form, Shellder will have a red jewel on its underside and a large horn on either side of its shell.

While in its crown form, Shellder will periodically release its venom into Slowking's brain. This is actually beneficial to Slowking, as it causes the Regal Pokémon's intelligence to increase. Higher quantities of venom intensify this effect, and Shellder is easily provoked into releasing more whenever Slowking yawns.

Shellder has two former signature moves. It can shoot spears of ice by using Icicle Spear, or clamp tightly to its opponents using Clamp. Despite its hard shell, clamping on to an opponent will reveal its vulnerable parts. Because of this, it will only clamp itself to a foe as a last resort. It swims around its sea floor home by rapidly opening and closing its shell while facing backward.